Process of manufacturing light-polarizing material



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@Price l y RBOCESS F LIGHT- u A rommzmGMArn-BIAL Edwin H, Land, Wellesley Mass., 'essi-gpm, by mesne nssiznment'x'-, to Polaroid Corporation,- Dover, Del., n corporation of Delaware Original lppllcation June 2, 19,35, Serial No.

83.040. 4Divided ena :his application December 10, 1937, Serial No. 179,113

.3U-alms. (cgb This invention relates to anew and improved process 'oi 'manufacturing light-polarizing material. 1

This application is a. division ot my co-pending application Serial No'. 83,040, illed June 2. 1936.A

which issued July 19, 1938, as U. S. Patent N o. 2,123,902.' `and which is s continuation in part of my co-pendng application Serial No. 72.501, nled April 3, 193s, which issued July 19,1938, es U. S. Patent No. 2,123,901. i v

An object of the invention is to provide a method oimaking a suspension ot crystalline, birefringent, needle-like particles with their needle axes oriented in substantial parallelism in s substantially isotropic suspending medium having an index of refraction equal substantially to one of the indices of the oriented crystalline particles.

.l tropic suspending medium 15.

Still further objects are to provide a. process forr the suspension oi' minute particles 'ot urea in a suspending medium. to provide such o. process wherein 'the suspending medium may comprise a set plastic; to provide such a process wherein the suspending medium may comprise a' set plastic and plasticizing materiel: to provide Suche' process wherein the' suspending mediumv may have an index of refraction equal substantially to one of the indices of urea: to provide'such a process wherein the suspending medium 'may be produced in sheet-like form and wherein the orientation of the crystalline particles of urea may be' effected in connection with the formation of such sheet-1ike form.

A still further object o! the invention is to vide such a. process wherein the suspending :ne` dium may be adapted to protect the suspended crystals of urea from solution in water. j

is especially useful insconnection with systems o! For a Afuller .understanding of the ris-ture and objects of the inventi'omreierence should be had v ized bea'm and the other as a spread or and differently polarized. beam. Such eproduct automobile headlight vglareelimination.

A still further object of thc'invention is to provide 'suche process wherein the suspension of urea is eected in a cellulosic material, such for example -as cellulose acetate or cellulose acetate propionate, or cellulose acetate butyrate. or ethyl cellulose.

A still further object of the invention is to provide such a, process wherein the urea particles may be suspended in a synthetic resin such as a p l ucized poiymerized'vinyl compound, such for example as Ylnyiite.

Other objects of the invention will in .part be obvious and will inpart appear hereinafter.

The invention accordingly comprises the' several steps and the relation o! one or more of such steps with respect to each ot the others,'tvl1ich are exemplified in the following detailed disclosure, and the scope of the application of which 5a.

win be .indicated m the claims,

cellulose acetate propionate'to 600 c. c'. o( ethyl This invention contemplates the. provision of a method of making a cheap, durable, manufactured product o i the type more generally described in my said cfs-pending applications, and more specically this invention contemplates the provision ot a method 'of making such a product comprising a, suspension of needleelike crystals of urea in any suitable suspending medium, the

yneedle'sxes-of the urea crystals being oriented to substantial parallelism, and the index .of refrac- -tion of the suspending medium coinciding .a.p--`

proximatelywith one of the indices oi refraction of urea. preferably with the lower or ordinary' In one form oi the invention asolution of urea in methanol in the proportions of, vfor example, 25 grams of urea, in 10 c. c. of methanol. is

formed at a temperature of lapproximately F, and this solution ls added toa solution of cellulose acette propionate in'ethyl ncetste in the proportions, for example, of .one pound oi acetate.'3 20 c. c. ci dimethyl phtlmlate being added to this mixture, and the mixing continued at n. temperature of pproximately 120' F. until.

a clear.' transparent, homogeneous marshes been secured. In this condition. the crystals of urea.

are dissolved in the solvent. The mass is then -cooled toA about room temperature, lor example',

by the arpplicationor eoldwater to the Jacket of the mixer, andin the proces; of pooling. minute,

lneedle-l1ke particles' ot urea. arelyprecipitated out within the mass, which gradually takes -on s i cloudy. 'milky appearance. The cooled mass may 'teen be plaeed 'in-suitable euer. aimedim-1- placed in an 'extrusion press and extruded inthe form of sheets. the extrusion acting simultaneously to orient the precipitated needle-like crystals of urea with ytheir needle'axes in substantial parallelism. Since the basal. section ofthe urea crystals is square, no further orientation beyond' the orientation of the needle axes oi' the crystals Occurs.

Upon evaporation oi the solvent, the extruded sheet assumesproperties lsuch thatthere is no tendency i'or the Aurea to go back into solution,

ciating it in close proximity to the iight source in a head-lamp or similar'device.

The addition to the mix of the dimethyl-' phthalate or other plasticizer acts with the cellulosel acetate propionate to give an index ot= refraction to the dried suspending medium which is approximately equal to the ordinary or lower index of urea, i. `e 1.481.

While extrusion has' 'been described as` the method of ori'enting the needle-like particles. it is understood that any of the other methods o! orienting may beemployed, such for example as rolling. stretching. smearing. or the like. Thus. the material with the particles therein may 'oe smeared by a putty ignite or other iiat blade. or even a brush. onto a at surface.

While' cellulose acetate proplonate' has been described as the suitable' suspending medium, it is to be understood that other media-may be employed, for example cellulose acetate or cellulose acetate butyrate, or vinyl acetate. or the conjoint polymer of vinyl acetate'and vinyl chloride, sold as "Vlnylite," or ethyl cellulose. When some o1 pending medium, the-process ma?. il desired. be

Acarried out in the manner previousLy described,

or because Voi' the solubility o! -ethyl cellulose in methanol a. modified proeessmay be employed.

may be somewhat more readily mixed with the remaining solution of ethyl cellulose in, for example, ethyl acetate.

AWhere vinyl acetate Le 'employed' as the suspending medium, .the process may follow the stepsl outlined in connection-with the use of cellulose acetate propionate as the suspending medium.

Where the conjoint polymer 'of vinyl chloride and vinyl'acetate is `employed as the suspending medium. abminim'u'm vazr'ioui'itoi' methanol should be'employed, and best results are obtained where no methanol is used and where dry .urea crystals are directly introduced linto the heated solution ot the polymer in the solvent employed, which may be. lor example, acetone or ethyl acetate.

Wherever solvents have been mentioned it il t be understoodv that the solvent s'pecied is not the only one which may be used, the, only ie quireinent being thatthe urea be sufficiently soluble in the vheated solvent to give an adequate precipitation oi crystalline needle-like particles in methanol at room temperature may be em-' these-materlals are employed, it may be found desirable -to use other solvents than ethylacetate or other plasticz'frs than ibutyl-phthalate or dimethyl phthalate. For example; di lJut.ylplfithal--4 ate is not -a. suitable plasticizer for cellulose acetate, but dimethyl-phthalate is.

A modiication of the process of this invention may comprise the introduction into asolutionof ceilulosic material, such forexample as cellulose acetate propionate, of the urea crystals in crystal form. When this modication o( the process is employed, the cellulose acetate propionate is dissolved in ethyl acetate in the manner previously mentioned. 'A small amount Otmethanol may be added to the ethyl acetate at the time of solu'- tion, i! desired. It is also convenient to add the plasticizer, ior example, dimethylphthalate, initlalLY.. The mixture of' cellulose acetate proupon cooling. Preferably the precipitation upon cooling should beso-complete that vfurther precipitation as the sheeted material dries is negli gible optically.l

While theprocess has been deseribedas employing in one form heated solutions oi the ma.-

terials used, it is' to be understood that ,the proc-- pioyed and may-be introduced, into the mixture of the plastic the solvent and the plasticizer at room temperature. 1nthis modicatlon of the process, the solvent for the plastic Vs hou-ld have so low a solubility for urea at room temperatures .as to cause the precipitation of the 'urea that was dissolvedin the methanol upon mixing. When the materials are mixed in this manner. care should -be taken to atoid the 'evaporationof the methanol on the surface of the mixbeiore ade-1 quate mixing has 4been accomplished.

While urea has. been mentioned as the crystalline compound employed, it will -be' obvious that other suitable -birefringent crystalline materials pionate, the solvents and the plasticizer may be heated to atemperature'of -aroundl20 F.. and

the desiredV quantity of urea crystals directly introduced thereinto. til the crystals have been dissolved and uniformly distributed throughout the4 mass. The

- process may then be continued in the manneroutlined above.

Where cellulose acetate is employed as the plastic suspending medium.' it is desirable-to add i an excess of urea crystals or an excess oi solution Mixingshould continue .un

- in the following manner:

The incident beam is resolved into two comas the suspending medium, the processf may follow substantially the steps Apreviously described.

Where ethyl cellulose, which has a blret'ring- Vence of about .0.01 and which may be considered as optically anisotropic, is Aemployed as the .ausw

may be used with notable .solvents and suspending media without departing from the-scope of the invention. l I! the sheet-likematerial is Iormed'by-extrusion, a suitable process may employ Vshaped Jaws having an aperture of about 0.012 inch and the material may be extruded'therefrom undern pressure of say about 25 pounds per square inch.

It will be obvious that these dimensions and pressures are by no means-criticakand thattheyfmay The sheet-like material of the present invention Iimctions .to polarize a transmitted light ponents, ior one of which the sheet is suistanf' tlally like a. sheet of glass. Thlsls'the oomprienf vibrating in a plane such ,that Ithe index ol the suspending medium and the index of the suspended crystalline Particles for `vibra .tions in that e iight, and wai be polarized.' 'niej otuefeem'po. nent, i. e., the component-vibrating ina direction for which the index of the suspending medium diiers from the index o! the suspended crystal.- line material, will be transmitted as a spread or diffused' beam, diderentl'y polarized. as the, suspended crystals may preferably be needle-shaped with their needle axes oriented.

v .this spread or diffused beam will be spread or diiused in a band, assuming that a. parallel orcollimated beam is impinging upon the sheet. lying in theplane of the specularly transmitted -beam ando! substantially the same height as that beam. For the' purposes of this invention, this type oi spreading ordiiusion will be termed cylindrical scattering, wherein the direction of orientation of the crystals in the material is parallel to the axisof the cylinder', and -the-scatter is along radii normal to this axis.

The sheet-like material of the present invention is exibla It may be rolled or bent or formed'. Where a plasticizer not-miscible with water, such 'as dibutyl-phthalate, is employed', the

vsheet-lille material is substantially water-prooi. even though urea crystals are themselvesvery soluble in water.

The product of the present invention may be adapted to polarize the extremes of the spectrum and may even be adapted to polarize the ultra violet and infra red, where the crystals employed are of suiliciently great diameter; i.' e., larger in diameter than the wave length of the radiation polarized. It is to be noted that the product of Inasmuch useof a plasticizenand thelanunatlon might be effected byemp-Ioyins as thebocdina or cement vinyl' acetate piasticizeu with 'dibutyiphthalate.

Speaking generally, .the two previously discussed, i. e., these-called cold method and the method oi forming theprod-uct 'by means o1 a process employing heatpr.ovide means for controlling the crystal size of the suspe'n-ded particles. The so-called cold process gives a larger crystal size than. does the' process employing heat,

'. and where a large crystal lis desired the cold procthe present invention polarizes by transmitting e one component and by scattering the other. Hence, even though it be employed as a polarizer for the infra red, it does not heat up through po larization of those rays.

The product o i the present invention is colorless and imparts no color characteristic to the transmitted polarized beam. For-the specularly transmitted component, the'product of this invention is absolutely colorless, lwhen examined with incident colorlesslight. Furthermore. the light lost through absorption in the-crystalline material is negligible, since, for example, in a sheet 0.0110 in thickness. the total thickness of the crystalline materialv traversed by any ray of the specularly transmitted beam may not exceed 0.0005 inch. Modern plastics are so transparent that the light lost through absorption therein is negligible. It follows, therefore, that the product oi the present invention transmits the specular or non-diiused component with a very high efliiciency and with negligible loss through absorption.4

It will beunderstood that the product o f the present invention may, if desired, be laminated,

' as for example between sheets ot glass'or other lightetransmitting protective media. In the process oi laminating, care should be takenv to avoid the use of temperatures so high thatthe crystals' tend to dissolvel in the suspending medium. Where no plasticizer is used in the suspending 1nedium, hi'gher temperatures may be employed and the finished product may be subjectedin use to higher temperatures than. where plasticizer is present. Where the finished product is to be subjected to high temperatures, the sheetingshould preferably contain no plasticizer and the lamination should be efiected by 'means of a cement or bonding material containing a plastlcizer which is a non-solvent for the sheeting. For examplathe sheeting material might` embody a distribution ess is preferred.

So also, alteration in the selection of the sus-V n pending medium may cause a-l-terat-ionin the'crystal size in the product of the present invention. For example, cellulose 4acetate propion'ate as o. suspending medium will tend to give asomewhat larger lcrystal than cellulose-acetate. and cellulose acetate butyr'ate when employed will tend to re'- sult in the iormation of still larger crystals.

So also, the selection of the solvent may result in alteration 'of the crystal size and shape. For

example, crystals formed .with ethylcellulose and acetone as the plastic and solvent tend tobe very long and thin with respect to crystals formed for example, with cellulose acetate butyrate and ethyl acetate.

it will be understood that apreierred wndition will be onen/herein the index of refraction for the suspending medium exactly matches the' desired index of refraction -of the suspended crystalline particles for all wave lengths utilized Il this condition cannot be eil'ected with the suspen.....g medium .and crystalline material em-.

ployed, the index of refraction of the suspending medium should preferably be controlled to exactly match the index oi the suspended particles for that wave length in the center -oi' the band of wave lengths which it is desired to use.

It will be obvious 'that the, index of the sus pending medium may be controlled Within reasonable vlimits by varying .slightly the Ypro'porticins oi' plastic and plasticizer and by a proper seiecfraction matching the upperiindex of 'the suspended particles. For example, where urea is employed as the suspended crystalline Athe index of refraction oi theinatrix should approximate LGQS.. Wherry gives the upper index of urea as 1.602, and' Bol-land gives the'upper index as 1.61.

A 'suitable `suspending' medium or'tnatrix may comprise. for example, benzyl cellulose, which.

may be .considered as substantlallyisotropic, havingindices ot e 1.567 and o 1.57. This ma@ teria] may be mixed with enough of the material sold under the trade name Arenen a chlori nated dlphenyl,V to'give the desired index. The i mixture may be accomplished' by dimming' in benzene, and the urea may he added to the mixture in the manner previously indicated.

o! y,crystals oiurea in cellulose acetate Without the For a further consideration or this problem. 'A

reference is-again made to .my an in v plications Serial Nos. 12,501'and 83,040,whereln a discussion of the desirability of matching one or the other index is to be found. In thespecification and claims, the term-"isotropic" shall be deemed to include media which are actually slightly birefringent, but in which the indices of refraction are bot-h so close in value to one of the indices 'of the crystals, as compared.

with the large difference between the two indices of the crystals. that the birefringence oi the sheet does not materially alter the differential scattering oi the ordinary andextraordinary. componente.

It should be noted Iurthermore, that the pro'cess of orienting the opticaxes of thesuspended crystalline material, as for example byextrusion smearing. and the like. will simultaneously so orient the .axes of birefringence of the matrix .refraction of oriented crystals Sof said bireiringthat it will ,not resolve the transmitted beam into additional components.

Since certain changes may be made in carrylng. out the above process without departing from the -scope of the invention, it is intended that all matter contained 'in theabove description or shown in the accompanying drawing .shall be interpreted as illustrative and not in a limiting sense. z

It is also to be understood that the following claims are intended to cover an of the generic and specific features of the invention herein described. and all statements of the scope oi the inventlonwhich, as a matter of language, might be said to fall therebetween.

Having described my invention, what I claim l as new and desire to secure by Letters Patent is:

`solution of a transparent plasticized plastic forming therein a solution of'a'bl.' 'efrineent crystalline material soluble therein .only when Y heated. cooling said plastic'with its solute to cause the formation-in said plastic ot a multiplicity of small crystals of said bireiringent inaterial, said plasticized plastic, when cooled to approximately roomV temperature, having an index oi refraction for light vibrating in a precie-v term-lned direction approximating the index of ent material for light vibrating in said direction, and orienting Vsaid crystals .within said plastic by' subjecting saidplastic to a field oi force to which `said crystals are responsive.

4. The process comprising forming a heated solution 0I a transparent plasticized plastic wherein urea is soluble only when heated, incorporating a solution of urea in said plastic, cooling said plastic with its urea content to cause the formation in said plastic ofv a multiplicity and substantially orlenting. the said crystals within the said plastic by subjecting the plastic to a eld. of force to which the crystals. are

responsive; l

2. The process comprising forming amixture of a solution oi urea and a solutionof a plasticized Y transparent plastic which when hard has an index of refraction for lightzvibrating in a predetermined direction coinciding substantially with the index o! refraction ot oriented crystals of 'urea for light vibrating in said direction causing the formation within said plastic of crystals o! urea by evaporation of theV solvent theretor..

hardening said plastic, and substantially orienting the said crystals within the said plastic by subjecting the plastic to a field of iorce to which thecrystalsareresponsive.

3. Ihe process comprising forming a heated of small crystals of urea. said. plasticized plastic.- when cooled to approximately room temperature.

having an index of refraction i'or light Vvibrating in a predetermined direction approximating the index of refraction of oriented 'crystals OI .urea for light vibrating in said direction, andyorienting said c.'ystals within said piastic by subjecting said plastic to a eld oi force to which said crystals are responsive. l

5. The process compling Iorming asolution in a. volatile solvent of a mixture of a transparent plastic-and a plasticizer therefor, said e mixture when hard .having an index of refraction for light vibrating in a predetermined direction approximating the index of refraction of oriented crystals of a soluble birefngent crystalline me.- terial for light vibrating insaid direction, adding to said solution and mixing therewith a solution of said birefringent material in a volatile solvent, removing lthe last mentioned volatile solvent to cause the formation within said plastirized plast-ic of a multiplicity of birei-ringent crystals, and orienting the-said crystals within the said plasticized -plastic by subjecting the said plastic to a iield o! forcerto `which the-crystals are responsive.

6. 'I'heprocesscomprising-forming a solution in a volatile solvent lof a oi' a transparent plastic and' aplasticizer therefor, said mixture when hard having' an index of v'xeiractim for iight vibrating in a predetermineddirection approximating .the index o i' refraction of oriented crystals of -urea for light vibrating insaid direction.

' adding to said solution and mixing therewith a solution of urea in a volatile solvent, removin the said volatile solvent to cause the-formation within said plasticlzed plastic of a multiplicity o! urea crystals, andorienting the said crystals within the said plasticized plastic bysubjecting the said plastic to a-fneld o! -Iorce to-which the crystals are responsive.

` EDWIN H. LAND. 

